This invention is generally related to a high density optical disk and a method of high density recording on an optical disk. Specifically, it relates to an optical disk which records on both lands and grooves with a reduced track pitch and the recording method.
For high speed data processing, high density optical disks have attracted attention. With the ISO standardization of 5.25-inch and 3.5-inch disks for optical and phase change schemes for overwriting, these disks can be expected to have even more widespread use in the future. Recently, the DVD (digital video disk) standardization such as SD standardization is about to be finalized. This standardization is expected to accelerate the use of optical disks in the area of multi-media.
In these optical disks, grooves and lands are used to guide a laser spot emitted from a pickup of a read/write system to the data. These grooves and lands are formed in a spiral from the center of the medium to its outer circumference. These grooves are called guide grooves. More specifically, as defined in the ISO standard, the recessed portions, which are more distant, as viewed from the pickup, are referred to as lands, and the raised portions closer to the pickup, are referred to as grooves.
Using a land recording method, the groove widths of from about 0.3 xcexcm to about 0.6 xcexcm, and the groove depths of about xcex/8(n) to about xcex/4(n) wherein xcex is the wavelength of a laser beam used during input and output operations, and n is a refractive index of substrate. Although the standard track pitch is 1.6 xcexcm, narrower track pitches are used to increase the density of the recorded data.
When using an optical pick up having an object lens of about 0.5 to about 0.6 aperture numerical (NA), the impact of the narrow track pitch on the undesirable simultaneous reading of data from adjacent tracks (hereafter referred to as the optical crosstalk) becomes critical and the tracking error signal is also negatively affected for accurate tracking.
For the optical disks currently on the market, the width W of substrate surface grooves is defined as W=(Wtop+Wbottom)/2, where Wtop is the width at the top of the groove, and Wbottom the width at the bottom. The groove depth is defined as the height of the substrate surface groove from the bottom of the groove to the top (step difference).
Various methods have been tried in an effort to increase the recording density of such optical disks, such as making the laser beam spot smaller by reducing the wavelength of the light source so as to read data at a higher recording density. There is a limit, however, in reducing the spot size due to the wavelength of semiconductor laser that can be used for a light source. In general, short wavelength laser has problems in forming a desirable beam shape as well as in an insufficient output level.
Other efforts to provide a higher recorded data density resulted in the proposal of a technology called magnetically induced super-resolution (MSR). This technology is capable of reading high-density recorded data using a currently available light source wavelength and a spot size. The MSR technology makes use of the temperature distribution of the recording medium inside an area under the light spot (due to a combination of the heating of the medium by the laser and the rotating motion of the medium) to mask a portion of the signal on the medium falling under the light spot so that it will not be detected as the read signal. This masking makes the effective aperture area from which a signal is read smaller than the laser spot size, thus making it possible to read higher density data.
The Front Aperture Detection (FAD) scheme, which is one way of implementing the MSR technology, will be briefly described, in reference to FIG. 2. FIG. 2(a) is a plan view of a FAD-type MSR disk 31, and FIG. 2(b) is a cross-sectional view of Section Ixe2x80x94I of FIG. 2(a). The FAD optical disk 31 has three magnetic layers: a recording layer 32, made of TbFeCo; a cutoff layer 33, made of TbFe; and a readout layer 34, made of GdFeCo. The signal is read from the readout layer 34. In the initial state as shown in FIG. 2(b), because coupling force is readily exchanged between the adjacent layers, the orientations of the magnetic field follow the magnetization of the recording layer 32, which stores the data as indicated by recording marks 38. During a read operation, an external magnetic field Hr is applied. When a relative position is established between a readout light spot 35 and a medium 31, as shown in FIG. 2(a), a temperature differential is created between a front low temperature area 36 and a rear high temperature area 37 under the light spot 35.
When the high temperature area 37 reaches the Curie temperature at which its magnetization is obliterated of the cutoff layer 33, the coupling of readout layer 34""s magnetization to recording layer 32 (through cutoff layer 33) is diminished, causing the magnetization of readout layer 34 to invert to align with the magnetization of the external magnetic field Hr. In FIG. 2(b), the magnetized direction of location A is inverted.
In the high temperature area 37, the magnetization of readout layer 34 always exhibits a constant state regardless of the presence of a recording mark 38 and is a mask without contributing to the readout signal. On the other hand, the signal is detected only in a low temperature area 36 where its recorded state is maintained. Thus, the low temperature area 36 serves as an effective signal detection aperture of the laser spot. This enables the system to only read recorded mark 38a in the area 36. In this FAD technique, the masking conditions are determined by the Curie temperature of the cutoff layer 33. Thus the MSR disks are manufactured fairly easily by controlling the composition of this cutoff layer. Techniques other than FAD have been proposed for the MSR disks. These include Rear Aperture Detection (RAD) and Center Aperture Detection (CAD), in which the high temperature portion of the spot is the aperture, and the remaining spot area is masked. RAD and CAD MSR disks have a smaller aperture because it is the higher temperature portion of the spot (the temperature distribution resulting from illumination by the readout laser) that is an aperture, and the lower temperature portion that is a mask. In contrast to this, in MSR disks using the FAD technique, the crescent shape of the aperture (low temperature area 36 within readout spot 35) renders it impossible to prevent leakage of signals from adjacent tracks.
The land-groove recording method has been also proposed for a high density recording. In contrast to the method in which data is recorded on either lands or grooves, the land-groove method increases the recording density to a half track pitch from a full track pitch by recording data on both lands and grooves. For example, if the center-to-center distance between adjacent lands (or grooves) and the next adjacent lands (or grooves) is 1.4 xcexcm, the proposed land-groove technique increases the data capacity, by reducing the track pitch to 0.7 xcexcm.
In this technique, an appropriate groove depth substantially reduces optical crosstalk or simultaneous data read from adjacent grooves (or lands). In addition, the center-to-center distance between lands (or grooves) of 1.4 xcexcm provides a sufficient space required to maintain for a tracking error signal.
However, crosserase or heat crosstalk is observed in recording or erasing information on tracks when the temperature of the adjacent tracks rises due to the heat from a laser beam. The higher temperature erases information on the adjacent tracks. Optical disks and the phase change schemes use thermal recording technique. In these optical disks, the shorter the distance between adjacent tracks, the more likely heat travels into adjacent tracks. Accordingly, it is inevitable for the crosserase problems to arise.
As illustrated in FIG. 14, a laser beam is irradiated onto an optical disk while modulating the peak intensity at Pp to the bottom intensity at Pb to form marks and to erase the recorded marks afterwards. To erase the recorded marks, a laser beam is irradiated onto the optical disk from a DC light source as illustrated in FIG. 15. At this time the intensity of the laser beam is determined at Pe. If the marks were originally recorded with the peak intensity at Pp, the erasure beam intensity of Pe should be able to erase the marks.
However, a variety of deviations for recording data originates from focusing or tracking servo mechanisms, contaminated lenses, changes in optical properties with time, contaminated optical disks, and various properties of optical disks due to temperature, humidity, atmospheric pressure, dust in air, etc. The intensity of the laser beam fluctuates as these factors change. As a result of these deviations, a laser beam intensity of Pe at Pp cannot effectively erase data and requires a greater intensity. In other words, the various factors mentioned above may increase Pp to a higher value or decrease Pe to a lower value. In this case, the recorded marks are recorded as physically larger marks as the intensity of the laser beam for erasing decreases. Accordingly, the recorded marks are insufficiently erased to leave a trace of recorded marks on the disk. This may cause recording and reading errors.
The relation between Pe and Pp at the cross-erasure is measured by the method described below. (1) Set a 130 mm optical disk having a groove depth of 75 nm and rotate it at a linear velocity of 9 m/sec. This recording reproduction system is equipped with an optical pickup using the wavelength of 680 nm, NA (numerical aperture) of 0.55, and the wave front aberration of 0.04 xcex (rms value). The system records data on the optical disk such that both the length of marks and the distance between marks are 1.2 xcexcm. The bottom intensity Pp of the laser beam is set to 0.8 mW and the recording field is set to 350 Oe. In addition, the peak intensity Pp is the value showing the minimum level for the 2nd-order harmonic frequency when the recorded marks are read and the read signals are put into a spectrum analyzer; (2) Next, data is recorded on a round of land (1 track) on the optical disk such that both the length of marks and the distance between marks are 0.64 xcexcm while modulating the intensity between the peak intensity Pp and the bottom intensity Pb of 0.8 mW. The magnetic field for recording is set to 350 Oe. Then, the recorded marks are read and the read signals are put into a spectrum analyzer to measure carrier level. (The initial carrier level is indicated as Co); (3) Then, a around of groove (1 track) adjacent to the inner circle of a round of land where the above marks are recorded and each a round of groove (1 track) adjacent to the outer circle are erased for 10 spinnings at Pe, the laser beam intensity erasing as illustrated in FIG. 7; (4) Next, the land on which marks are recorded is reproduced and the reproduced signals are put into a spectrum analyzer to measure a carrier level. (After cross-erasure, carrier level is indicated as Ce); (5) Change the Pe value. Repeat (3) to (4) until Coxe2x88x92Ce=0.5. Obtain Pe/Pp at that time. Then, Pe/Pp is obtained in the same manner as described in the above from (1) to (5) for erasing lands adjacent to recorded grooves.
In the conventional optical disk with guide grooves, the heat crosstalk determines the track pitch. The track pitch for the conventional optical and phase change type disks is limited to about 0.8 xcexcm. The track pitch for the optically modulated overwritable optical type is limited to about 0.9 xcexcm or about 1.0 xcexcm. A narrower track than the above mentioned dimension had been considered impractical.
Since the performance of an optical disk is dependent on not only the heat crosstalk but also noise from the substrate, it had been difficult in prior art to manufacture an optical disk without the above-described two undesirable effects.
Prior art teaches a technique to increase the groove depth in order to reduce thermal crosstalk. In other words, increasing the groove depth increases the distance heat propagates before reaching the adjacent tracks so as to reduce the undesirable effect of heat. For example, Japanese patent 6-223421 discloses that when the groove depth is increased up to 130-280 nm from a conventional range from about 40 to about 90 nm, the track pitch can be narrowed.
This does not mean that any depth of 100 nm or more is acceptable. First, the preferable land or groove reflection coefficient of is 0.5 or more in order to maintain a desirable read signal level. However, the land or groove reflection coefficients vary based on the groove depth. Some groove depth values even lower the readout signal level, causing a data readout error. In addition, although a preferable push-pull signal modulation factor is larger than 0.2 in order to maintain an accurate tracking capability, some groove depth values lower the push-pull signal modulation factor to cause mistracking, slow access, and erroneous erasure.
The current invention is directed to resolve the above and other described problems and to provide an optical disk having reduced thermal crosstalk and noise from the substrate. In addition, this invention is directed to a method of reading and recording high-density data on an optical disk.
In order to solve the above and other problems, according to one method of manufacturing optical disks for substantially reducing cross-talk during input and output operations to and from the optical disks each having grooves and lands, data being stored on both the grooves and lands, the method includes the steps of: a) specifying a track pitch between the grooves and the lands; b) selecting a depth of the grooves d in relation to the lands to satisfy a first relation, d=xcex/(an) where a is a parameter variable, xcex is a laser wavelength used during the operations and n is a refraction index of an optical disk substrate; and c) further selecting the depth d so as to make the cross-talk equal to or less than xe2x88x9225 dB while the first relation in the step b) is maintained.
According to a second aspect of the current invention, a method of manufacturing optical disks for substantially reducing cross-talk during input and output operations to and from the optical disks each having grooves and lands, data being stored on both the grooves and lands, includes the steps of: a) specifying a track pitch between the grooves and the lands; and b) selecting a depth of the grooves d in relation to the lands to satisfy a first relation, d=xcex/(an) where a is a parameter variable, xcex is a laser wavelength used during the operations and n is a refraction index of an optical disk substrate, the parameter ranges from 2.8 to 3.4.
According to a third aspect of the current invention, a method of manufacturing optical disks for substantially reducing cross-talk during input and output operations to and from the optical disks each having grooves and lands, data being stored on both the grooves and lands, includes the steps of: a) specifying a track pitch between the grooves and the lands; b) selecting a depth of the grooves d in relation to the lands to satisfy a first relation, d=xcex/(an)+m xcex/(bn) where a and b are parameter variables, xcex is a laser wavelength used during the operations, m is an integer and n is a refraction index of an optical disk substrate, wherein the parameter a satisfies 5.2xe2x89xa6axe2x89xa66.8 while the parameter b satisfies 1.8xe2x89xa6bxe2x89xa62.1, m being a natural number.
According to a fourth aspect of the current invention, a method of manufacturing optical disks for substantially reducing thermal cross-talk during input and output operations to and from the optical disks each having grooves and lands including the steps of: a) specifying a track pitch between the grooves and the lands to be equal or less than 1.1 xcex where xcex is a laser wavelength used during the operations; and b) selecting a depth of the grooves d in relation to the lands to satisfy a first relation, d=xcex/(an) where a is a parameter variable and n is a refraction index of an optical disk substrate.
According to a fifth aspect of the current invention, a method of manufacturing optical disks for substantially reducing thermal cross-talk during input and output operations to and from the optical disks each having grooves and lands includes the steps of: a) specifying a track pitch between the grooves and the lands to be equal or less than 0.96 xcex where xcex is a laser wavelength used during the operations; and b) selecting a depth of the grooves d in relation to the lands to satisfy a first relation, d=xcex/(an) where a is a parameter variable and n is a refraction index of an optical disk substrate.
According to a sixth aspect of the current invention, a method of manufacturing optical disks for substantially reducing thermal cross-talk during input and output operations to and from the optical disks each having grooves and lands comprising the steps of: a) specifying a track pitch between the grooves and the lands to be equal or less than 0.81 xcex where xcex is a laser wavelength used during the operations; and b) selecting a depth of the grooves d in relation to the lands to satisfy a first relation, d=xcex/(an) where a is a parameter variable and n is a refraction index of an optical disk substrate.
According to a seventh aspect of the current invention, a method of manufacturing optical disks for substantially reducing thermal cross-talk during input and output operations to and from the optical disks each having grooves and lands comprising the steps of: a) specifying a track pitch between the grooves and the lands to be equal or less than 1.1 xcex where xcex is a laser wavelength used during the operations; and b) selecting a depth of the grooves d in relation to the lands to satisfy a first relation, d greater than xcex/(4n) where n is a refraction index of an optical disk substrate.
According to an eighth aspect of the current invention, a method of manufacturing optical disks for substantially reducing thermal cross-talk during input and output operations to and from the optical disks each having grooves and lands, data being stored on both the grooves and lands, including the steps of: a) specifying a track pitch between the grooves and the lands at a predetermined width ratio of the grooves and the lands; and b) selecting a depth of the grooves d in relation to the lands to satisfy a first relation, d greater than xcex/(4n) where xcex is a laser wavelength used during the operations and n is a refraction index of an optical disk substrate.
According to a ninth aspect of the current invention, an optical disk for substantially reducing cross-talk during its input and output operations to and from the optical disks, includes: grooves and lands located on the disk for storing data on both the grooves and the lands, a predetermined distance between the grooves and the lands being defined as a predetermined track pitch, wherein the grooves having a depth d in relation to the lands, wherein the depth is related to a parameter variable a, a laser wavelength xcex which is used during the operations and a refraction index n of an optical disk substrate in a relation as (d) greater than xcex/(an), the depth d is further determined so that cross-talk is equal to or less than xe2x88x9225 dB while the relation is maintained.
According to a tenth aspect of the current invention, an optical disk for substantially reducing cross-talk during its input and output operations to and from the optical disks, including: grooves and lands located on the disk for storing data on both the grooves and the lands, a predetermined distance between the grooves and the lands being defined as a predetermined track pitch, wherein the grooves having a depth d in relation to the lands, wherein the depth is related to a parameter variable a, a laser wavelength xcex which is used during the operations and a refraction index n of an optical disk substrate in a relation as (d) greater than xcex/(an), wherein the parameter a ranges from 2.8 to 3.4.
According to an eleventh aspect of the current invention, an optical disks for substantially reducing cross-talk during input and output operations to and from the optical disks, including: grooves and lands located on the disk for storing data on both the grooves and the lands, a predetermined distance between the grooves and the lands being defined as a predetermined track pitch, wherein the grooves having a depth d in relation to the lands, wherein the depth is related to parameter variables a, b and m, a laser wavelength xcex which is used during the operations and a refraction index n of an optical disk substrate in a relation as d=xcex/(an)+m xcex/(bn), wherein the parameter a satisfies 5.2xe2x89xa6axe2x89xa66.8 while the parameter b satisfies 1.8xe2x89xa6bxe2x89xa62.1, m being a natural number.
According to a twelfth aspect of the current invention, an optical disks for substantially reducing thermal cross-talk during input and output operations to and from the optical disks, including: grooves and lands located on the disk, a predetermined distance between the grooves and the lands being defined as a predetermined track pitch, wherein the predetermined track pitch is equal to or less than 1.1 xcex where xcex is a laser wavelength used during the operations, the grooves having a depth d in relation to the lands, wherein the depth is related to a parameter variable a and a refraction index n of an optical disk substrate in a relation as d=xcex/(an).
According to a thirteenth aspect of the current invention, an optical disks for substantially reducing thermal cross-talk during input and output operations to and from the optical disks, including: grooves and lands located on the disk, a predetermined distance between the grooves and the lands being defined as a predetermined track pitch, wherein the predetermined track pitch is equal to or less than 0.96 xcex where xcex is a laser wavelength used during the operations, the grooves having a depth d in relation to the lands, wherein the depth is related to a parameter variable a and a refraction index n of an optical disk substrate in a relation as d=xcex/(an).
According to a fourteenth aspect of the current invention, an optical disks for substantially reducing thermal cross-talk during input and output operations to and from the optical disks, including: grooves and lands located on the disk, a predetermined distance between the grooves and the lands being defined as a predetermined track pitch, wherein the predetermined track pitch is equal to or less than 0.81 xcex where xcex is a laser wavelength used during the operations, the grooves having a depth d in relation to the lands, wherein the depth is related to a parameter variable a and a refraction index n of an optical disk substrate in a relation as d=xcex/(an).
According to a fifteenth aspect of the current invention, optical disks for substantially reducing thermal cross-talk during input and output operations to and from the optical disks, including: grooves and lands located on the disk, a predetermined distance between the grooves and the lands being defined as a predetermined track pitch, wherein the predetermined track pitch is equal to or less than 1.1 xcex where xcex is a laser wavelength used during the operations, the grooves having a depth d in relation to the lands, wherein the depth is related to a refraction index n of an optical disk substrate in a relation as d greater than xcex/(4n).
According to a sixteenth aspect of the current invention, an optical disks for substantially reducing thermal cross-talk during input and output operations to and from the optical disks, including: grooves and lands located on the disk for storing data on both the grooves and the lands, a predetermined distance between the grooves and the lands being defined as a predetermined track pitch, wherein the predetermined track pitch is also specified by a predetermined width ratio of the grooves and the lands, the grooves having a depth d in relation to the lands, wherein the depth is related to a laser wavelength xcex used during the operations and a refraction index n of an optical disk substrate in a relation as d greater than xcex/(4n).